The present invention relates to a water level detecting apparatus of the float type for reliably detecting water level at two positions, namely at a predetermined upper limit and a predetermined lower limit, and more particularly to such an apparatus which is suitable for automatically controlling underwater pumps.
Conventional water level detectors for controlling the operation of underwater pumps, as shown in FIGS. 8A and 8B comprise a float 1" upwardly or downwardly movable along a guide 2" in accordance with the water level, a magnet 5' provided on the float 1" and a reed switch 4' disposed within the guide 2". When the float 1" reaches a predetermined upper limit or lower limit, the magnet 5' closes the reed switch 4' as shown in FIG. 8A. The switch 4' is opened when the magnet 5' moves away therefrom with a fall or rise of the water level as seen in FIG. 8B. The operation of the reed switch 4' is transmitted to the motor circuit of the pump by way of a transformer and power relay. With such an apparatus, the magnet 5' moves away from the reed switch 4' to stop the pump when the water level lowers from the upper limit even if slightly, so that the range over which the water level is controllable is very small and indefinite. The apparatus therefore has the drawback that waves on the water surface cause chattering. Further for controlling the operation of the underwater pump, there is the need to use two water level detectors for starting the pump at the upper limit and stopping the pump at the lower limit. This requires a complex arrangement outside the pump and is costly. Further since it is impossible to incorporate into the detecting apparatus the transformer and power relay which requires a large space, these components must be housed in the head cover of the pump motor. The cover needs to be correspondingly larger, consequently rendering the motor large-sized. If it is attempted to modify a non-automatic pump to an automatic pump, there arises the necessity of preparing a new head cover. The labor and expenditure then needed will be unjustifiable. Accordingly it is practically impossible to incorporate a water level detector into an existing non-automatic pump to render the pump automatically operable, and automatic pumps equipped with such a detector must be designed initially for automatic operation.